The present invention generally relates to an electrohydraulic motor for use in hydraulic shovels, asphalt finishers, machine tools, and cranes. More particularly, the invention relates to an electrohydraulic motor enabled to relieve superfluous operation oil during stoppage of a driving operation without using excessive energy.
As shown in FIG. 7, in a hydraulic drive system 700 using a conventional electrohydraulic motor, operation oil stored in a tank 710 is caused by a pump 720 to flow through a main oil passage, and then reaches a spool valve 741 provided in the electrohydraulic motor 740. The operation oil having reached the spool valve 741 is caused by movement of the spool valve 741 to flow through one of two communicating oil passages 742a and 742b. Then, the operation oil is supplied to a cylinder block (not shown) of a hydraulic actuator 743. The operation oil supplied to the cylinder block provides a pressure to a piston (not shown). In response to a sliding operation of the piston, an output shaft 743 of the hydraulic actuator 743 is rotated. When the output shaft 743 is rotated, the operation oil having provided the pressure to the piston receives a pressure from the cylinder block. Subsequently, the operation oil having received the pressure from the cylinder block flows through the other communicating oil passage 742a or 742b. Finally, such operation oil reaches the spool valve 741. This operation oil having reached the spool valve 741 is returned to the tank 710 through a return oil passage 750.
The rotation direction of the output shaft 743a is determined according to which of the two communicating passage 742a and 742b the operation oil having reached the spool valve 741 is supplied to, that is, which direction the spool valve 741 moves. The spool valve 741 and a drive shaft 744a of a pulse motor 744 are connected to each other so that each of the spool valve 741 and the drive shaft 744a is rotatable. Further, a rotation shaft 745 is connected to the drive shaft 744a. A first threaded shaft 746 is screw-connected to the rotation shaft 745. The first threaded shaft 745 engages a second threaded shaft 747 in such a way as to be perpendicular thereto. Thus, the spool valve 741 is moved by rotation of the pulse motor 744 according to the difference in the number of revolutions between the drive shaft 744a and the output shaft 743a. 
Incidentally, the hydraulic actuator 743 is provided with a revolution speed changing member 748 that comprises a receptive capacity changing member 748a for changing the operation oil receiving capacity of the hydraulic actuator 743, a cylinder 748b connected to the receptive capacity changing member 748a, a higher-pressure oil selection valve 748c for drawing operation oil from one of the communicating oil passages 742a and 742b, which has a pressure higher than that of the other communicating oil passage, and a switch valve 748d for switching the connection between the cylinder 748b and the higher-pressure oil selection valve 748c. 
To prevent the pumped operation oil from returning to the pump 720, a check valve 749 is provided in the main oil passage 730 that connects the pump 720 to the spool valve 730. Further, when the internal pressure of the main oil passage 730 becomes abnormally high, the operation oil contained in the main oil passage 730 is discharged into the tank 710 through a relief valve 760.
Furthermore, as illustrated in FIGS. 8 and 9, the conventional electrohydraulic motor has a cup-like first casing 50, and a second casing 52 fastened and fixed to the first casing 50 with bolts 52. A main oil passage 50a, a return oil passage 50b, and two communicating oil passages 50c and 50d are formed in the first casing 50.
The output shaft is rotatably supported in the first casing 50 and the second casing 52 by bearings 55 and 54, respectively. A first helical gear 56 is rotatably connected to the spool valve 59 through the bearings 54 and 55. The first helical gear 56 and a second helical gear 57, which is fixed to the output shaft, engage each other so that axes of the gears 56 and 57 are perpendicular to each other.
Annular grooves are formed in an outer peripheral portion of the spool valve 59 in such a manner as to extend in the circumferential direction thereof. When the spool valve 59 moves in the direction of a rotation shaft 58 of a pulse motor 60, the annular grooves are connected to a drain oil passage, the main oil passage 50a, the return oil passage 50b, and the communicating oil passages 50c and 50d. Further, when gears formed on the shaft 58 move, the main oil passage 50a and the return oil passage 50b are connected to the communicating oil passages 50c and 50d. 
The drive shaft 58 is connected to a drive shaft 61 of the pulse motor 60, and screw-connected to the second helical gear 57. Thus, the second helical gear 57 can be moved in the direction of the drive shaft 61 by rotation of the drive shaft 61 of the pulse motor 60 (see JP-A-2000-213502).
However, in the case of the hydraulic drive system 700 using the conventional electrohydraulic motor, when the spool valve is in a neutral position, the operation oil supplied by the pump stagnates in the main oil passage. When the operation oil stagnates in the main oil passage, the internal pressure of the main oil passage increases. When the internal pressure thereof becomes high, the pump supplies the operation oil into the main oil passage by utilizing a pressure that is higher than the internal pressure of the main oil passage. Incidentally, the pressure for operating the relief valve is set at a very high value. Thus, the internal pressure of the main oil passage reaches the set pressure of the relief valve. Consequently, the conventional electrohydraulic motor has encountered a problem in that very high energy is consumed only for relieving (hereunder referred to as xe2x80x9cbleeding offxe2x80x9d) the operation oil, which is supplied by the pump, from the relief valve.
Moreover, in the case of the conventional electrohydraulic motor, when the output shaft of the hydraulic actuator is rotated by an external force, the hydraulic actuator operates as a pump. When the hydraulic actuator operates as a pump, the operation oil is sent from one of the two communicating oil passages to the other communicating oil passage. At that time, in the case that the spool valve and the hydraulic actuator constitute a closed circuit, and that the hydraulic actuator operates as a pump, the pumped amount of operation oil is not replenished to the communicating oil passage from which the operation oil is pumped out. Consequently, a cavity is produced (hereunder, such production of a cavity will be referred to as xe2x80x9ccavitationxe2x80x9d) in the communicating oil passage, from which the operation oil is pumped out, especially, in the conventional electrohydraulic motor adapted to perform mechanical feedback. Thus, the conventional electrohydraulic motor has encountered drawbacks caused in the hydraulic actuator owing to the cavitation, for example, a problem that the hydraulic actuator becomes uncontrollable.
Furthermore, in the conventional electrohydraulic motor, the return oil passage and the drain oil passage are not separated from each other. The drain oil passage is connected to the return oil passage. Consequently, pressure oil from the drain oil passage flows into the return oil passage that is in a high pressure condition. Thus, the conventional electrohydraulic pump has a problem that an oil seal provided at an output-shaft-side portion of the hydraulic actuator is ruptured.
Accordingly, an object of the invention is to provide an electrohydraulic motor enabled to discharge superfluous oil without consuming very high energy. Another object of the invention is to provide an electrohydraulic motor enabled to prevent the return oil passage from being put into a very high pressure condition.
To solve the aforementioned problems, according to an aspect of the invention, there is provided an electrohydraulic motor, that comprises a hydraulic drive means for rotating an output shaft by a pressure of operation oil, an electric drive means for rotating a drive shaft according to an inputted electric signal, a drive switch means, connected to the hydraulic drive means, to a main oil passage for leading operation oil supplied from the exterior, and to a return oil passage for leading operation oil to the exterior, for switching connection between the hydraulic drive means and each of the main oil passage and the return oil passage, and a connection switch means, which is connected to the main oil passage and the return oil passage, for changing connection between the main oil passage and the return oil passage. In this electrohydraulic motor, the drive switch means responds to rotation of the drive shaft to thereby switch between a drive position, in which each of the main oil passage and the return oil passage is connected to the hydraulic drive means, and a neutral position in which connection between the hydraulic drive means and each of the main oil passage and the return oil passage is disconnected. Further, the connection switch means is adapted to connect the main oil passage to the return oil passage in response to an operation of the drive switch means, and also adapted to break the connection between the main oil passage and the return oil passage.
With such a configuration, the main oil passage and the return oil passage are connected to each other by the connection switch means when the drive switch means is in the neutral position. Thus, the operation oil supplied to the main oil passage is returned to an operation oil supply source. Consequently, there is no need for bleeding off superfluous oil, which stagnates in the main oil passage, by a relief valve. Therefore, there is no necessity for consuming very high energy so as to activate and operate the pump.
Further, a flow control means for sending the drive switch means a necessary amount of operation oil supplied from the exterior and for diverting the remaining operation oil downstream can be connected to a bypass oil passage that connects the main oil passage to the connection switch means. When the drive switch means is in the drive position, the connection switch means breaks the connection between the main oil passage and the return oil passage. Thus, the operation oil sent from the main oil passage provides a pressure to the flow control means. Then, the flow control means changes the state of the flow of the operation oil in such a way as to let a necessary amount (that is, a predetermined amount needed for enabling rotation of the hydraulic actuator) of the operation oil, which is sent from the exterior, run in the direction of the drive switch means. Incidentally, the remaining operation oil is let to run downstream. On the other hand, when the drive switch means is in the neutral position, the connection switch means connects the main oil passage to the return oil passage. Thus, the operation oil, which is sent from the main oil passage, and the operation oil, which provides the pressure to the flow control means, run together toward the return oil passage. At that time, in the case that the destination of the diverted operation oil is another electrohydraulic motor, the operation oil, which is superfluous to one of the electrohydraulic motors, can be used for driving the other electrohydraulic motor. Consequently, energy for activating and operating the pump can effectively be used.
Preferably, the electrohydraulic motor according to the invention further comprises a cavitation preventing means connected to a communicating oil passage for passing operation oil through between the drive switch means and the hydraulic drive means, and to the return oil passage, and adapted to supply operation oil from the return oil passage to the communicating oil passage when the pressure of the communicating oil passage is lower than that of the return oil passage.
With such a configuration, when cavitation occurs in one of the communicating oil passages, operation oil is supplied to the communicating oil passage, in which the cavitation occurs, from the return oil passage by the cavitation preventing means. Thus, the electrohydraulic motor of the invention can avoid drawbacks caused in the hydraulic actuator owing to the cavitation, for example, a problem that the hydraulic actuator becomes uncontrollable.
According to another aspect of the invention, there is provided a hydraulic driving method, according to which torque is obtained by supplying operation oil to a hydraulic drive means enabled to generate torque by a pressure of operation oil, from the exterior, comprising the steps of performing a circulating process of supplying operation oil to the hydraulic drive means from the exterior and of returning the operation oil from the hydraulic drive means to the exterior, and performing a disconnecting process of inhibiting operation oil from circulating between the hydraulic drive means and the exterior. According to this hydraulic driving method, the operation oil supplied form the exterior is returned to the exterior together with operation oil outputted from the hydraulic drive means in the disconnecting process. Furthermore, only a necessary amount of the operation oil supplied from the exterior is fed to the hydraulic drive means in the circulating process.
With such a configuration, in the disconnecting process, the operation oil sent from the exterior is returned together with the operation oil, which is sent from the hydraulic drive means, to the exterior. In the circulating process, the operation oil sent from the exterior is supplied only to the hydraulic drive means. Thus, there is no necessity for bleeding off superfluous operation oil by using the relief valve. Moreover, this eliminates the need for consuming very high energy so as to activate and operate the pump.
Furthermore, according to another aspect of the invention, there is provided an electrohydraulic motor that comprises a hydraulic actuator for rotating an output shaft by a pressure of operation oil, an electric drive means for rotating a drive shaft according to an inputted electric signal, a spool valve, connected to the hydraulic actuator, to a main oil passage for leading operation oil supplied from the exterior, and to a return oil passage for leading operation oil to the exterior, for switching connection between the hydraulic drive means and each of the main oil passage and the return oil passage by responding to rotation of the drive shaft, a first threaded shaft connected to the spool valve, a second threaded shaft connected to the output shaft and engaged with the first threaded shaft so that said first threaded shaft is perpendicular to said second threaded shaft, and a separation wall provided in such a manner as to surround the second threaded shaft. In this electrohydraulic motor, a part of the separation wall, which part is provided at the side of the second threaded shaft, serves as a drain oil passage. Moreover, a part of the separation wall, which part is provided at the side opposite to the side of the second threaded shaft, serves as the return oil passage.
With such a configuration, the pressure oil outputted from the drain oil passage does not flow into the return oil passage that is in a high pressure condition. Thus, the internal pressure of the return oil passage does not become very high. Consequently, an oil seal provided at an output-shaft-side portion of the hydraulic actuator can be avoided from being ruptured. Therefore, the electrohydraulic motor can be applied to a series circuit to which the hydraulic actuator is series-connected, and HST (Hydrostatic Transmission) circuit for controlling the hydraulic actuator according to the discharge rate of the pump.
The present disclosure relates to the subject matter contained in Japanese patent application No. P2001-342395 (filed on Nov. 7, 2001), which is expressly incorporated herein by reference in its entirety.